1. Field of the Invention
The present invention relates to a distillation system and, more particularly, to a simplified and automated distillation system which is capable of recovering a purified solvent from a solvent mixture.
2. Description of the Related Art
Distillation is widely used, both in the laboratory and on a large industrial scale, to separate boilable liquids (solvents) present in a liquid mixture. During distillation, the liquid mixture is at least partially vaporized and collected separately from the residue. For the most part, the effectiveness of separation by distillation is determined by differences in the boiling points of the solvents which make up the liquid mixture. In other words, it is more difficult to separate by distillation, liquid mixtures having solvents with close boiling points.
Most distillation systems have several common features. The common features are illustrated schematically in embodiments of a laboratory-type distillation system, FIG. 5, and a batch-type distillation system, FIG. 6. The common features are:
1. A heated area 510, 610 where volatile components are evaporated to form vapors.
2. A reflux section 520, 620 where vapors are xe2x80x9cwashedxe2x80x9d by cooler liquid returning to the evaporation area.
3. A condenser section 530, 630 where the xe2x80x9cwashedxe2x80x9d vapors are condensed to form a condensate.
4. Single or multiple discharge ports 540, 640 for the collection and/or recirculation of the condensate.
Distillation is a very old technology, and was developed for the distillation of alcohol spirits, perfumes, and oils. Today, the distillation equipment is relatively more sophisticated, and even a small laboratory-type unit has the capability to provide high-purity separations of closely-boiling solvents. The modern equipment achieves such high-purity separations by a variety of techniques including spinning band techniques, the use of many theoretical plates (stages), high ref lux ratios, multiple collection ports, and water-cooled spirally-arranged condenser tubes.
For simple distillation tasks, however, such high-performance equipment has many disadvantages. For example:
1. It can be very expensive.
2. It requires careful attention during operation.
3. An experienced operator is generally required.
4. It often contains glass and other breakable parts that can cause hazardous conditions should they break or leak.
5. It requires frequent maintenance and parts replacement.
6. Special work areas and facilities (water and 220 volt outlets, hoods, etc.) normally are required.
7. Operating cycles can be quite long.
Moreover, in many small clinical and research laboratories, cleaning facilities, and machine shops, only relatively small amounts of one or two solvents are used on a regular basis, and there thus has been little incentive to recycle solvents. The relatively high costs of the modern distillation equipment, its operation, and the required manpower and working space have discouraged widespread use.
At the same time, however, there have been mounting pressures for solvent recycling both for cost and environmental reasons. Substantial costs are associated with the purchase and disposal of the solvents. Therefore, recycling the solvents would generate considerable cost savings, and would have beneficial environmental effects.
An object of the invention is to provide a distillation system which is simple in structure and operation.
Another object of the invention is to provide a simplified and an automated distillation system which is capable of recovering a purified distillable component from a starting mixture having relatively close boiling points.
Still another object of the invention is to provide a simplified and an automated distillation system which is small, operator-independent, inexpensive, safe, durable, virtually maintenance-free, requires no special facilities, and is usable in virtually any location.
Still another object of the invention is to provide a simplified method for automatically recovering a purified distillable component from a starting mixture having relatively close boiling points.
The above and other objects of the invention are accomplished with a distillation system whose design resulted, in part, from the following simplifying specifications for its operation:
1. In a preferred embodiment, one volatile component can be purified in a single distillation cycle.
2. A minimum acceptable purity of the recovered distillable component is specified rather than just trying to obtain as high purity as possible. The specified purity is that which is needed for the particular application.
3. Distillable component yields are dependent upon the purity specification. The total collected fraction will contain some portions which have less than the specified purity and some portions which have more than the specified purity, but, on the average, meets the purity specification.
The distillation system according to the invention includes a transition tank. When a starting mixture including at least distillable component is distilled batch-wise, there exists a separation problem at the point when one fraction is ending and the next fraction is beginning. To minimize the mixing of one distillable component with another distillable component or with waste component(s) from the starting mixture during this transition, rather sophisticated techniques are used conventionally, including the use of spinning band, vacuum, large number of theoretical plates, high reflux ratios, and multiple collection ports.
The transition tank eliminates the need to separate these transition mixtures into their separate components. Instead, these mixtures are simply put aside and stored in the transition tank until the current distillation cycle is completed. The transition tank is then drained into the evaporation tank at or prior to the beginning of the next distillation cycle. Volume-wise, these transition mixtures are usually not particularly large. At two to three theoretical plates and a specified purity level of greater than 99.5%, they tend to be about 10-25% of the starting volume. At five to ten theoretical plates and a specified purity level greater than 99.5%, the transition mixtures tend to be about 1-5% of the starting volume.
A first advantage of the distillation system according to the invention is that high purities can be obtained with relatively small numbers of theoretical plates. During the distillation of a distillable component, the purity of the collected product is highest at around the midpoint of the collection cycle, and it decreases toward both ends of the cycle (i.e., beginning and end). If a minimum acceptable purity is established for this middle cut, it then is possible to determine the minimum number of theoretical plates that are needed to produce the minimum specified purity. For example, in a laboratory environment in which a purity of 99.0% can be more than adequate, only two to three theoretical plates are required with the distillation system according to the invention.
With the distillation system according to the invention, simply by increasing the number of theoretical plates from two or three, the yields (amounts) of the purified distillable component at a specified purity are increased. As a corollary, if a desired yield is specified, then the minimum number of theoretical plates which are required for a particular distillation run can be determined. By reducing the number of plates, equipment size, run times, cooling and the like are minimized. For the distillation system according to the invention, preferred results are obtained most often with theoretical plates numbering from one to about twenty. By definition, xe2x80x9cone platexe2x80x9d consists of simply boiling and condensing a solution with no refluxing.
As noted earlier, the end portions (end cuts) of each fraction that do not meet the minimum purity specifications are simply put aside in the transition tank and reprocessed in the next run. The point at which each cut is made is controlled by the temperature sensors which, in turn, control the different reflux and collection solenoid valves. The closer that the temperature switch point is to the actual boiling point of a distillable component (both below and above it), the smaller the yield. The actual temperature switch points that are used for a particular distillable component are those that provide the specified purity with the maximum yield.
A second advantage of the distillation system according to the invention is that good purities and yields can be obtained at relatively low ref lux ratio numbers. The reflux ratio is defined as the volume of condensate that is recycled back through the reflux column divided by the volume of condensate that is diverted for collection. This can be thought of as being a measure of the amount of xe2x80x9cwashingxe2x80x9d given to vapors coming up from the evaporation tank. Higher ratios indicate more xe2x80x9cwashingxe2x80x9d which, in turn, results in higher levels of purity. The disadvantages of high reflux ratios are that bigger equipment is needed; product is produced more slowly; run times are increased; the size of heaters and the usage of electricity are increased; and cooling requirements are increased, often substantially.
Because of the above disadvantages of using high reflux ratios, it is an advantage to have reflux ratios that are as low as possible. Thus, in the distillation system according to the invention, once the minimum number of theoretical stages is set, the reflux ratios can be reduced to a point where the amount of recirculating condensate becomes just sufficient to meet the minimum specified purity. For the distillation system according to the invention, the preferred ratios range from about 0 to about 5. By definition, a distillation unit with only one theoretical plate has a reflux ratio of zero. In practice, the reflux ratios can be controlled in the system simply by controlling the size of the orifice in the partial reflux solenoid valve (see element 130 of FIG. 1).
The reduction in reflux ratio thus results in a reduction in the size of the equipment, as demonstrated by the present invention. The reduction in size permits the use of smaller heaters and ordinary 110 volt wall current, and a change from a water-cooled system to an air-cooled system. Furthermore, operating times are reduced.
A third advantage of the distillation system according to the invention is that an automated operator-less system can be constructed. The system requires only temperature sensors, solenoid valves, and optional timers, if the distillation is limited to the purification of the desired components to pre-specified purity levels. In such systems, the components in a liquid mixture are collected into at least two fractions: a purified distillable component fraction and a waste fraction. In Example 1 (below), the components are collected into one of three fractions: a lower boiling waste fraction, a purified distillable component fraction, and a higher boiling waste fraction. In cases where the desired distillable component is the highest boiling component, a third fraction could still exist and it would consist of a relatively non-volatile component like suspended solids. In other cases where the desired distillable component is the lowest boiling component in a mixture (see Examples 2 and 3 below), only two fractions are obtained because there is no lower boiling fraction. With more complex mixtures, it is possible to obtain multiple purified component fractions and/or multiple waste fractions.
It is to be noted that the contents of the transition tank are not a final or ultimate fraction, and hence, they were not included in the above discussions, and should not be considered to be a separate fraction.
The automated system replaces the much more complex systems which employ intricate computer controlled operations, continuous measuring systems, variable heating programs, trained operators, and the like.
A fourth advantage of the distillation system according to the invention is that ordinary atmospheric air pressures can be used for most of the practical and commonplace types of separations. Reduced pressure (vacuum) or greater than atmospheric pressure conditions can be used, if desired, but such conditions can require complicated and expensive equipment which is capable of maintaining these vacuums or pressure conditions. A preferred embodiment of the present invention therefore operates at ordinary atmospheric pressure.
A fifth advantage of the distillation system according to the invention is that small xe2x80x9chold-upxe2x80x9d volumes are not always required for good separations. xe2x80x9cHold-upxe2x80x9d volume refers to the volume of distillable component(s) retained in the reflux and condenser portions of the system during the distillation. The reason for this simplification is the existence of the transition tank which allows xe2x80x9chold-upxe2x80x9d volumes of distillable component to be reprocessed in the next run. As a result, the reflux column is much simplified in the distillation system according to the invention.
The distillation system according to the invention also includes thermal sensors, which may be thermal switches, thermocouples, or the like, optimally located between the upper one-third of the reflux column and the beginning of the condenser unit.
By comparison, traditional multi-product laboratory-type distilling equipment can employ:
1. A single temperature read-out device (e.g., a thermometer) which usually is located in the evaporation tank, and whose function is to provide identification information to the operator as to which component is being boiled off at a particular point in time.
2. A fraction collection system (manual or automatic) that is dependent upon a monitoring system (physical and/or chemical measurements) to xe2x80x9ctellxe2x80x9d the collection system when to change to a different collection port, or to a different collection container.
The thermal sensor located between the upper one-third of the ref lux column and the beginning of the condenser unit, replaces the monitoring sensors of the above-described more-complicated monitoring systems. In the distillation system according to the invention, the thermal sensor is, in a sense, a measuring sensor. The temperature in this section of the distillation system, at any one time, is a reflection (indirect measurement) of the components that are present. Thus, by adjusting the temperature set points of the thermal sensor, the purity and yield of the collected component(s) can be controlled.
The distillation system according to the invention also includes additional thermal sensors located at other positions, but these are used for different purposes. For example, one of the sensors in the evaporation tank is used to turn off the external heater, and the sensor at the top of the condenser unit is used for safety purposes.
The distillation system according to the invention further includes a reflux column and a condenser tube, both of special designs. The reflux column has a plurality of flow distributors and flow centralizers spaced axially within the column housing. Irregularly-shaped solids, e.g., roofing nails, are interdisposed in the gaps between the axial positions of the distributors and the flow centralizers. The special design improves xe2x80x9cwashingxe2x80x9d and thus reduces the overall height of the reflux column.
The condenser unit has an inverted design, which allows for a very rapid condensation of vapors even though the condenser has a relatively small and compact shape. This inverted design of the condenser tube has several important features:
1. It reduces the xe2x80x9cdeadxe2x80x9d inner volume of the condenser unit so that vapors are always quite close to the cooling surfaces (both inner and outer). The helical inner fins also have the effect of forcing the vapors upward and toward the outer wall, which has the greatest cooling capability. The fins, in themselves, provide additional cooling surfaces while allowing for good drainage of condensate.
2. This design allows for the use of condensing units with large cooling surface areas which can be a function of large diameters since surface area equals pi times diameter times length. In the absence of an inverted section, a large diameter tube would have a large xe2x80x9cdeadxe2x80x9d inner volume that would have poor cooling properties.
3. The inverted structure provides two large cooling surfaces with a relatively low manufacturing cost and a greater ease of construction. More complex and convoluted cooling structures have been described (e.g., helical condensing tubes), but these structures are much more difficult to make and they cost proportionately more.
Additional objects and advantages of the invention will be set forth in the description which follows. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.